1. Field of the Invention
The present invention relates to a DC to DC buck converting controller, and more particularly a DC to DC buck converting controller with programmable output voltage.
2. Description of Related Art
FIG. 1 is a schematic diagram of a conventional DC to DC buck converting circuit. The DC to DC buck converting circuit comprises a controller 10, two switches M1 and M2, an inductance L, a capacitance C, a bootstrap circuit BS and a voltage divider VD. The voltage divider VD detects an output voltage of the buck converting circuit and accordingly generates a feedback signal FB. The controller 10 turns the switches M1 and M2 on/off according to the feedback signal FB, so as to make the DC to DC buck converting circuit to convert an input signal Vin into an output voltage Vout which is stabilized at a preset output voltage, as well as provide an output current Iload.
The controller 10 is packaged in a package, and comprises a comparator 12, a on-time period circuit 14, and a logic circuit, which has a logic control circuit 16 and two gate driving units 18, 20. The comparator 12 generates a feedback control signal according to the feedback signal FB and a reference voltage Vref, which is generated inside the controller 10. An on-time period of the on-time period circuit 14 is determined by the input voltage Vin and the output voltage Vout, and the on-time period circuit 14 generates a constant on-time signal according to the feedback control signal. The logic control circuit 16 determines conduction timing and cut-off timing of the switches M1 and M2, and generates two control signals Sl and Su respectively via the gate driving units 18 and 20 to turn on and off the switches M1 and M2. The switch M2 is a N-type MOSFET. For avoiding that the gate driving unit 20 in the controller 10 cannot generate a signal which is high enough to turn on the switch M2. The bootstrap circuit BS is used to supply a sufficiently high voltage to the gate driving unit 20.
The constant on-time period circuit 14 adjusts the constant on-time period according to the input voltage Vin and the output voltage Vout to make the DC to DC buck converting circuit operate in a quasi-constant frequency. Therefore, an electromagnetic interference (EMI) generated by the switches M1 and M2 can be easily filtered out, regardless of the levels of the input voltage Vin and the output voltage Vout in different applications.
Compared with a conventional converting controller with error amplifier structure, the DC to DC buck converting controller with on-time structure has a better transient response. FIG. 2 shows waveform diagrams when a loading driven by the conventional converting circuit with on-time structure is changed. At a tome point t1, the output current Iload is raised while the loading increases. During the interval from the time point t1 to a time point t2, the output voltage Vout is temporarily decreased due to that an increased output power provided by the converting circuit is not enough. After the time point t2, the output voltage Vout starts to be elevated and then reaches the original voltage level at a time point t3. The constant on-time period circuit 14 determines the on-time period in response to the input voltage Vin and the output voltage Vout. However, the output voltage Vout is lower than the original voltage level during an interval from the time point t1 to the time point t3, and so the on-time periods of cycles within the interval are shorter, which is a great disadvantage for transient response.